In recent years, development of a nonaqueous electrolyte battery, mainly a lithium ion battery, is being aggressively carried out. In a nonaqueous electrolyte battery, a microporous film (separator) is usually provided between positive and negative electrodes. The separator has a function of preventing direct contact between the positive and negative electrodes and passing an ion through an electrolytic solution held in micropores.
In order to enhance the cycle characteristics and safety of a nonaqueous electrolyte battery, improvement of a separator is studied. Recently, with the reduction in size and thickness of a portable appliance, reduction in size and thickness is also required of an electricity storage device such as a lithium ion secondary battery. On the other hand, in order to enable a portable appliance to be carried for a long period of time, an increase in the capacity of an electricity storage device by attempting to improve the volume energy density is being simultaneously persued.
Conventionally, the separator is required to have specific performance with regard to safety, such as the characteristics (fuse characteristics) of terminating a battery reaction immediately upon occurrence of abnormal heating, and property (short circuit characteristics) of maintaining shape even at high temperatures to prevent a direct reaction of a positive electrode material with a negative electrode material. In addition thereto, from the standpoint of making a charge-discharge current uniform and suppressing lithium dendrite formation, adhesiveness to an electrode is also required of the separator.
When the adhesiveness of the separator to a battery electrode is improved, a charge-discharge current is kept from becoming non-uniform and a lithium dendrite is less likely to precipitate, and as a result, the charge-discharge cycle life can be extended.
For example, in Patent Documents 1 and 2, it has been proposed to apply a slurry for a porous film onto an electrode or a separator and thereby bond/fix the separator to the electrode.
In Patent Documents 3 and 4, it has been proposed to further apply a polymer solution as a dot pattern layer onto a porous coating layer formed on a porous base material, thereby obtaining a separator, and bond/fix the dot pattern layer side of the separator to an electrode.